British patent specification GB 23 686 40 B describes an approach in which a dispenser for dispensing minute volumes is provided with capillaries with which a liquid that is to be dispensed is picked up by means of the capillary effect. In order to dispense the liquid held in the capillaries, an excess pressure is generated at one end of the capillaries. This can be done either individually or preferably for all of the capillaries together by connecting them to a shared pressure chamber.
A dispenser described here consists of a plurality of capillaries that are open on both sides, a source of compressed air, a valve for switching the compressed air on and off, a pressure chamber consisting of an upper part with a compressed-air inlet and a lower part in which the capillaries are held. The upper part and the lower part of the pressure chamber are sealed off with respect to each other, so that when the pressure chamber is charged with compressed air, the compressed air is distributed uniformly over the capillaries and the liquid contained in the capillaries is expelled.
A considerable disadvantage of such a capillary dispenser is that the capillaries are emptied in parallel by means of a shared source of pressure. In order to reliably empty all of the capillaries completely, it is necessary to work at a high pressure since capillaries that have already been emptied function as a “short-circuit” and the compressed air can escape from these capillaries with relatively little resistance. Typical operating pressures here are 15 to 25 psi or 1.0 to 1.7 bar.
The subsequently incoming air exits the capillaries at a speed of several meters per second and the expelled volume of air can amount to a few milliliters, depending on the size of the pressure chamber, on the pressure applied and on the number of capillaries. This fact considerably diminishes the usefulness of this technology.
Dispensing discrete drops onto a “smooth” surface such as, for example, a specimen slide, is very difficult or even impossible since the subsequently incoming air disperses the drops and distributes them over the surface. For this reason, the only option is to dispense the liquid into wells of so-called microtiter plates. However, there are limitations here as well; for instance, liquid can only be dispensed into empty wells since the subsequently incoming air can cause splashing in the wells that are already filled with liquid, thus leading to cross-contamination between the individual wells.
For this reason, such capillary dispensers are designed purely for dispersing liquid into empty wells. This, however, does not completely eliminate the problem of cross-contamination. The capillaries are never completely emptied right away. When the liquid is expelled, a liquid film remains on the inside of the capillary and it only moves slowly towards the end of the capillary as a result of the force of gravity and the subsequently incoming air. Here, due to the high flow rate of the air, the film is atomized into an aerosol. When sufficiently sensitive measuring equipment is employed, this aerosol can also be detected as cross-contamination due to the high concentration of active ingredient that is present there.
In the search for a solution to the above-mentioned problems, the person skilled in the art will also turn to the state of the art of the type known from liquid-handling systems equipped with pipettes.
The person skilled in the art of liquid-handling systems is familiar with so-called air-displacement pipettes that make use of a plunger-cylinder unit with which liquid can be picked up and dispensed by means of a pipette tip connected to said unit.
The pipette tip here is fastened to an opening in the face of the inner cylinder. The plunger is arranged inside the inner cylinder so that it can move on a shared axis and so as to be sealed off with respect to the inner cylinder. Raising or lowering the plunger inside the inner cylinder increases or decreases the free volume of the inner cylinder associated with the pipette tip, as a result of which a liquid can be picked up or dispensed via the pipette tip, corresponding to the change in volume.
If a capillary were to be installed instead of a pipette tip, the capillary effect would cause the capillary to already automatically fill up when it is dipped into the liquid since the free volume of the inner cylinder is quite large relative to the volume of the capillary. Therefore, it would not be necessary to lift the plunger in the inner cylinder in order to pick up liquid through the capillary.
The capillary, however, could be emptied according to the principle of air displacement in that the free cylinder volume is reduced by lowering the plunger in the inner cylinder, so that the air contained therein is expelled via the capillary.
If a plunger is provided for each individual capillary, the air displacement could be dosed in very fine increments. This allows the formation of a drop on the free end of the capillary that can be deposited systematically by making contact with, for instance, a specimen slide.
If the acceleration and the achievable speed of the plunger are high enough, a free-falling drop or liquid jet can also be dispensed contact-free. The pressure needed in this case for an individual capillary is smaller than when all of the capillaries are blown free together since the pressure does not have to be over-dimensioned as is necessary in view of the possibility of “short-circuits” of the type described in the state of the art.
Experiments have shown that the free volume of the inner cylinder displaced by the plunger and thus the amount of air subsequently flowing out of the capillaries was less than 50 μl and thus smaller by a factor of 20 to 200 than with a capillary dispenser in which all of the capillaries are connected to a pressure chamber.
This smaller volume of expelled air, with the concurrent lower pressure and thus lower dispensing speed, make it possible to dispense onto smooth surfaces as well as into vessels that already contain a liquid such as the wells of a microtiter plate. This also largely prevents the formation of aerosols.
When it comes to the above-mentioned advantages, as far as the form is concerned, the use of plunger-cylinder units that are each connected to a capillary seems to be evident to the person skilled in the art. The disadvantages associated with a capillary instead of a pipette, however, will preclude the person skilled in the art from pursuing such an idea.
After each dispensing procedure, the plunger must be returned to the initial position without inadvertently picking up any liquid in this process before it is ready to be used again for dispensing. This entails the risk that, if there is any residual liquid present in the capillaries or if the equipment is operated incorrectly (.e.g. the capillaries are still immersed in the liquid), liquid will be drawn up into the inner cylinder, and the function of the dispenser is no longer ensured.